The present invention relates to a method for forming a resist pattern on a semiconductor substrate during a process of fabricating a semiconductor integrated circuit device.
In a conventional process of fabricating a semiconductor integrated circuit device, a resist pattern is formed by a photolithography technique using ultraviolet radiation. How ever, since a greater and greater number of semiconductor integrated circuit devices are integrated on a single chip with an increasingly higher density in recent years, the significance of fine-line patterning technique is growing day after day. In order to cope with such demand, the wavelength of exposing radiation used for photolithography should be further reduced.
Nevertheless, the shorter the wavelength of exposing radiation is, the more seriously the accuracy is affected by rays reflected from the substrate underlying a resist film and a level difference of the resist film. To avoid these adverse effects, according to a suggested pattern forming method, a surface resolution process is performed by forming a masking layer over the surface of a resist film and patterning the resist film using the masking layer. In accordance with this method, ArF excimer laser light (wavelength: 193 nm) is used as exposing radiation and a chemically amplified resist is used as a resist material.
Hereinafter, a method for forming a positive resist pattern by exposing a chemically amplified negative resist to ArF excimer laser light will be described with reference to FIGS. 4(a) through 4(c) and FIGS. 5(a) and 5(b).
First, as shown in FIG. 4(a), the surface of a semiconductor substrate 1 is coated with a chemically amplified negative resist, thereby forming a resist film 2 thereon. The resist contains: polymers; an acid generator for generating an acid when exposed to radiation; and a cross-linking agent for cross-linking the polymers when heated in the presence of an acid. Thereafter, as shown in FIG. 4(b), the resist film 2 is exposed to excimer laser light 3 through a mask 4 having a desired pattern. As a result, an acid is generated from the acid generator in exposed portions 2a of the resist film 2.
Then, as shown in FIG. 4(c), when the semiconductor substrate 1 and the resist film 2 are heated, the polymers are cross-linked in the exposed portions 2a owing to the function of the cross-linking agent, because the acid has been generated there. On the other hand, the polymers are not cross-linked in non-exposed portions 2b of the resist film 2, because no acid has been generated there.
Next, as shown in FIG. 5(a), a silylation reagent 5, composed of dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied onto the entire surface of the resist film 2. As a result, a silylated layer 6A is formed on the surface of the non-exposed portions 2b of the resist film 2. However, no silylated layer 6A is formed on the surface of the cross-linked, exposed portions 2a of the resist film 2.
Subsequently, as shown in FIG. 5(b), the resist film 2 is dry-etched with an etching gas 7A, essentially consisting of O.sub.2 gas, using the silylated layer 6A as a mask, thereby forming a positive resist pattern 8A out of the non-exposed portions 2b of the resist film 2.
In order to meet the demand for further increasing the degree of integration of, and further decreasing the size of, semiconductor integrated circuit devices, a method for forming a fine-line resist pattern with a line width of about 0.1 .mu.m should be developed.
Thus, it is imaginable to use extreme ultraviolet radiation having a wavelength in the band of 13 nm or 5 nm, instead of ArF excimer laser light (wavelength: 193 nm), for the purpose of increasing the resolution of exposing radiation for a photolithography process.
Hereinafter, a pattern forming method, considered by the present inventors as a basis of this invention, will be described with reference to FIGS. 6(a) through 6(c) and FIGS. 7(a) and 7(b). In the method, a positive resist pattern is formed by exposing, to extreme ultraviolet radiation, a resist film made of a chemically amplified negative resist having the following compositions:
Polymer: poly(vinyl phenol) (1 g) PA1 Cross-linking agent: melamine (0.25 g) PA1 Acid generator: triphenylsulfonium triflate (0.04 g) PA1 Solvent: diglyme (5 g)
First, as shown in FIG. 6(a), a resist film 2 made of a chemically amplified negative resist and having a thickness of 0.5 .mu.m is formed on a semiconductor substrate 1. Next, as shown in FIG. 6(b), the resist film 2 is exposed to extreme ultraviolet radiation 9, having a wavelength on the band of 13nm, using a mask 4, thereby generating an acid from the acid generator in exposed portions 2a of the resist film 2.
Then, as shown in FIG. 6(c), the semiconductor substrate 1 is heated by a hot plate at 100.degree. C. for 60 seconds. As a result, the polymers react with the cross-linking agent owing to the function of the acid generated from the acid generator. Specifically, hydroxyl groups, which are cross-linking groups contained in the polymers, react with the cross-linking agent, whereby cross-linkage is generated in the exposed portions 2a of the resist film 2.
Subsequently, as shown in FIG. 7(a), a silylation reagent 5, composed of dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied onto the entire surface of the resist film 2. As a result, a silylated layer 6B is formed on the surface of the non-exposed portions 2b of the resist film 2. However, no silylated layer 6B is formed in the cross-linked, exposed portions 2a of the resist film 2.
Thereafter, as shown in FIG. 7(b), the resist film 2 is dry-etched with an etching gas 7B, essentially consisting of O.sub.2 gas, using the silylated layer 6B as a mask, thereby forming a positive resist pattern 8B out of the non-exposed portions 2b of the resist film 2.
However, the region of the resist film 2 where the polymers are cross-linked (i.e., the hatched regions with dense dots in FIG. 6(c)) covers not only the exposed portions 2a but also the non-exposed portions 2b. Accordingly, the size (i.e., width) of the silylated layer 6B is smaller than the size (i.e., width) of the non-exposed portions 2b of the resist film 2 as shown in FIG. 7(a). As a result, the size (i.e., width) of the resist pattern 8B is unintentionally smaller than the size (i.e., width) of the light-blocking portions of the mask 4 as shown in FIG. 7(b). Such a decrease in size accuracy of the resist pattern 8B causes various failures during subsequent process steps of a fabricating process of a semiconductor integrated circuit device.